These are the criteria radiologists use to determine if findings on obstetrical ultrasound are diagnostic of miscarriage vs. suspicious for but not diagnostic of miscarriage: https://www.acog.org/-/media/Practice-Bulletins/Images/pb200a.ashx. Embryo with crown-rump length of 7 mm or greater and no heartbeat is diagnostic of miscarriage. It used to be embryo with crown-rump length of 5 mm or greater and no heartbeat, but the guidelines were revised to be absolutely sure that the pregnancy is not viable. Sorry for your loss :(
This does happen! You are early and sometimes they can't see the heartbeat. Do they see the sac? I have to ask this, have they made sure you aren't having an ectopic pregnancy (which can be the result in high HCG numbers)? Yes, if you are miscarrying, your hcg numbers should drop. Let us know what happens and sorry for what you are going through. hugs